Studies of neural circuit function and development would be advanced considerably by tools capable of "dialing down" or "turning-off" neurotransmission from select cells in the awake, freely behaving mouse. Especially powerful would be tools capable of effecting inducible and reversible suppression of neurotransmission, and which could do so with great cellular specificity - where the choice of cell type to be silenced is based on combinatorial codes of gene expression and thus is highly selective for a particular neuron subtype. Such a tool would allow causal relationships to be defined between a highly select neural circuit, gene expression (concurrent or antecedent) and animal behavior. Further, the features of inducibility and reversibility would allow silencing to be triggered during discrete developmental periods followed by recovery;from this it would become possible, for example, to reveal periods of circuit development during which select neuron activity may be critical for later circuit health - without which mature circuit function may be compromised or decline prematurely. Thus developmental windows of vulnerability for genetic or environmental insult could be identified. Here we propose developing and testing four genetic tools - four independent mouse strains - each designed to offer these capabilities, but each likely to differ in the kinetics and efficiency of neuron inactivation and recovery and thus in the types of circuits and behaviors most suitable for study. We will build upon a prerequisite and exciting set of tools already established in the lab: we will take component elements from a set of alleles, ones that we have recently shown able to visualize and silence a wide range of neuron subtypes in highly selective fashion, and now incorporate along with these proven elements other sequences that should allow for inducible and reversible silencing. By engineering these elements into single broadly applicable alleles, we simplify the experimental paradigm with respect to both time and expense. The innovation of the proposed work lies not in an individual genetic element but rather in how these elements will be used together - the resultant tools should have the potential to dramatically change neuroscience research through their use by many investigators to study the function and development of virtually any circuit in the mouse nervous system. PUBLIC HEALTH RELEVANCE A fundamental challenge of contemporary neuroscience is to define causal relationships between the activity of a given brain circuit and a particular animal behavior, including the developmental and molecular events that tie this relationship together. Towards meeting this challenge (one defined by this RFA-MH-08-060), we propose to develop tools for visualizing and manipulating the development and activity of discrete neural circuits (molecularly defined) in the awake, behaving mouse or in developing embryos otherwise undisturbed in utero. If successful, these reagents will have the potential to radically advance numerous areas of basic and translational neuroscience research;indeed, our hope is that these tools, generated by a single lab and R21 funding mechanism, will be leveraged, through their use by many neuroscientists, to advance studies of virtually any neural circuit in the mouse nervous system.